Bacterial pneumonia represents a major cause of morbidity, mortality, and health care expenditure. Protective immunity against bacterial pathogens in the lung requires the generation of both innate immunity and acquired immune responses. Cells within the lung that are critical in bridging innate and acquired immune responses include macrophages and dendritic cells, which do so by expressing co-stimulatory molecules, presenting antigen, and by expressing type 1 cytokines and chemokines required for the development of cell mediated and humoral immunity. Bacterial components and cellular receptors that initiate this process have been incompletely defined. It has been known for some time that bacterial DMA serves as a potent activator of distinct leukocyte populations, including dendritic cells. More recently, it was shown that the immunostimulatory effects of bacterial DMA are attributable to unmethylated CpG dinucleotide motifs, and CpG induced-activation of host cells occurs through the toll like-receptor TLR9. The hypothesis to be tested in this application is that TLR9 contributes to the generation of effective immunity against both extracellular and intracellular bacterial pathogens of the lung. The Specific Aims of this proposal are as follows: 1) to determine the contribution of TLR9 to dendritic cell activation and effector cell function in response to extracellular (Klebsiella pneumoniae) and intracellular (Legionella pneumophila) bacterial pathogens in-vitro;2) to determine the time course of expression, the cellular sources, and role of TLR9 to antibacterial host in murine pneumonia due to extracellular and intracellular bacterial pathogens of the lung;3) to determine the effect of adoptive transfer of naive and antigen-pulsed DC on lung antibacterial host defense in WT and TLR9-deficient mice with bacterial pneumonia;and 4) to assess the effect of compartmentalized delivery of CpG motif-containing oligodeoxynucleotides on the generation of protective immunity in murine Klebsiella and Legionella pneumonia. The studies outlined will explore fundamental mechanisms that link innate and adaptive immunity in bacterial pneumonia and potentially identify new approaches to immunotherapy in the treatment of this disease.